Page 322 - Biosystems Engineering

Bioseparation Pr ocesses 299

snow; as solidification from a liquid melt, as in the freezing of water
to ice or the manufacture of large single crystals; or as crystallization
from liquid solutions. Good yield and high purity are two important
objectives in crystallization. Reasonable size uniformity is desirable
for washing, filtering, reacting with other chemicals, transporting,
and storing the crystals.
There are seven different classes of crystals depending on the
angles and lengths of the axes: triclinic, monoclinic, orthorhombic,
tetragonal, trigonal, hexagonal, and cubic.
In the formation of a crystal, two steps are required: (1) the birth
of a new particle (nucleation) and (2) the crystal’s growth to macro-
scopic size. In a crystallizer, crystal-size distribution is determined by
the interaction of the rates of nucleation and growth; the overall pro-
cess is complicated kinetically. The driving potential for both rates are
supersaturated, and neither nucleation nor growth can occur in a
saturated or unsaturated solution.

Crystal Formation
Supersaturation may be generated by one or more of the following three
methods: (1) if the solubility of the solute increases strongly with an
increase in temperature, as in the case with many common inorganic
salts and organic substances; (2) if a saturated solution becomes super-
saturated by the relative independence of temperature, as in the case
of common salt; supersaturation can be done by cooling or evaporation
of the solution; (3) if nearly complete precipitation of the product is
required, a new solute may be created chemically by adding a third
component that will react with the original solute and from an insoluble
substance (precipitation).
Types of Crystallizers
There are six types of crystallizers:
1. Tank
2. Scraped surface
3. Double pipe–scraped surface
4. Circulating liquid evaporator
5. Circulating magma vacuum
6. Continuous oscillatory baffle
Tank Crystallizers In this crystallizer, saturated solutions are allowed
to cool in open tanks. After a period of time, the mother liquid is
drained and the crystals are removed. Nucleation and size of crystals
are difficult to control. This process results in very high labor costs.
Scraped Surface Crystallizers In this type of crystallizer, an open
trough consists of a slow-speed agitator and a cooling jacket outside.